Sizing from square footage
The quickest BTU estimate uses floor area and a per-square-foot factor: about 20 BTU for cooling, and roughly 30 to 60 for heating depending on climate. This gives a sensible starting point for choosing an air conditioner or heater without a full load calculation.
Adjusting for real rooms
Treat the square-footage estimate as a baseline and adjust for the things it ignores. Add capacity for rooms with lots of sun, high ceilings, poor insulation or heavy use, and for kitchens where appliances add heat. For whole-home systems or unusual spaces, a professional Manual J load calculation gives the most accurate result.